A conventional motor generator system, as utilized for example in the aerospace industry, includes a brushless synchronous machine that generates multi-phase AC power from a rotating shaft, e.g., coupled to a gas turbine engine, and DC excitation. In addition to operating in a generator mode, the brushless synchronous machine operates as a starter (motor) to start the aircraft engine. Following a successful engine start the system initiates the generator mode.
Conventionally, motor controllers for applications requiring a controlled torque use discrete sensors to determine rotor position in a rotating machine. This technique, however, increases system complexity and decreases system reliability. The electric machine must have a sensor built in or attached mechanically to the rotor. Interfaces and wiring must be added for control (excitation) and feedback signals between the controller and the sensor. Typical sensors include resolvers, encoders, and the like. The location of the rotating machine could be far from the controller, creating the need for unwanted extra wiring in the system.
A conventional motor control system having a position sensor is shown in FIG. 1A. The primary components of the system include a power source 110, a controller 120, a motor generator 130 and a speed/position sensor 140. The controller 120 includes inverter control 126 that receives signals from the sensor 140 (e.g., speed/rotor position) and the motor generator 130 (e.g., current, voltage). These signals are used to control the main inverter 122 and exciter inverter 124, thereby providing a conventional closed loop system to regulate the torque/speed/current/voltage of the motor generator 130, as will be appreciated by those skilled in the art.
FIG. 1B illustrates a block diagram of a sensorless system. As is apparent from the block diagram, the sensor and related signals to the controller 120 are absent. Those skilled in the art will appreciate that this requires the controller 120 to process the rotor position/speed of the motor generator 130 to allow closed loop torque/speed regulation or to exclude certain control functions (e.g., torque control) or operate in an open loop mode.
Sensorless motor control techniques can increase system reliability and eliminate the need for extra wiring in the system. In addition these techniques eliminate the need for a discrete position sensor and also reduce the system cost. A sensorless motor control technique is a more flexible/adaptable solution for a motor drive system than one that relies on a separate position sensor. It is particularly valuable for an aircraft system where increased reliability and reduction of weight (e.g., through elimination of the sensor and additional wiring) are extremely important.
Motor controller applications in systems with existing electrical machines can use a sensorless motor control scheme. For example, sensorless control systems are advantageous in retrofit applications, where a sensor and appropriate wiring may be unavailable and not easily installed. Some of these systems have synchronous generators that can be used as a motor generator but they do not have discrete sensors. Additional applications for this technique include motor controllers in the environmental control systems, electric power systems, industrial drive systems, and the like.
U.S. Pat. No. 5,920,162 issued to Hanson et al. describes a system that utilizes feed through from the exciter winding of twice the fundamental frequency of excitation thereof which is detected synchronously in one of a plurality of stator phase windings of a main motor generator. The one of a plurality of stator phase windings is maintained in a non-commutated state during operation as a motor to determine rotor position of the main motor generator for control of commutation of current in all other commutated stator phase windings. The amplitude modulation of the voltage across each stator phase winding which is maintained in a non-commutated state represents the rotary position of the rotor of the main motor generator which is used to control commutation of current flow in an at least one and preferably all remaining commutated stator phase.
However, although the above-described system operates as in a sensorless mode, it requires that the position sensing take place only on the non-commutated stator windings. Accordingly, the position sensor emulation must shift from phase to phase as respective phases are commutated, which complicates the sensor emulation. Therefore, it is desired to have a sensor emulation technique and sensorless control system that truly emulates a continuous position sensor and is not dependent on the commutated state of the stator windings. Further, prior art sensorless systems fail to provide initial position sensing at standstill or low speed ranges that is necessary at start-up under high load torque of the motor generator.